The
Study on the Inheritance of the Natural Bobtail in Australian Shepherds:
Final Results
by
Gemi Sasson-Brickson
Reprinted from the May-June 2006 Aussie Times, with permission.
©2006-08 - Imagineer/Gemini Sasson-Brickson
All Rights reserved. All photographs and narratives from this web site are
copyrighted and can only be reproduced with written permission from the author.
***Complete data results available upon request.
Early in 2003, I launched a study on Australian Shepherd litters to help uncover the mode of inheritance of the natural bobtail trait in Australian Shepherds. Collecting this litter data over a period of three years (2003-2006) also provided valuable information on litter size and neonatal health. Litter data was collected from a total of 76 litters from North America, Europe and Australia.
Litter information was collected from breeders for the following:
1) Total number of pups born per litter
2) # of living pups
3) # of early deaths (premature, stillborn and by 8 weeks)
4) Abnormalities (presence of cleft palates, spina bifida, tail kinks, imperforate anuses, etc.)
5) Tail length of pups at birth
6) Tail length of parents at their birth (if known)
The data was then analyzed for various crosses (overall, NBT X NBT, Full-tail X NBT and Full-tail X Full-tail) for the following:
1) Avg. # of pups per litter
2) % Mortality and % Survival
3) Tail length
4)
# of pups with specific defects (kinked tails and imperforate anuses)
Following are a data table and a table of statistics, with the complete results from all litters in the study. (Please note that Litter #2 was not included in the results, since it did not meet the minimum of two pups per litter. I also calculated results for NBT X NBT litters both with and without Litter #1, which was born prematurely. Premature births can be attributed to a variety of factors, including hormonal imbalance in the bitch and uterine infections.) Litter # 8 (8a and 8b) was actually two litters born on the same date to the same dam. The bitch was bred on the same heat to two different stud dogs and the resulting offspring were DNA’d for parentage verification. For study purposes they were identified as two separate litters. Some Miniature Australian Shepherds litters were submitted for the study. Because of possible differences in the gene pool and the fact that Miniature Australian Shepherds are considered by many to be a separate breed, I did not include those litter submissions in these study results.
Statistics: Study of the
Inheritance of the Natural Bobtail in Australian Shepherds
Litter Size:
Total # Litters in Study = 76
Total # Pups born in this study = 566
Avg. # pups per litter overall in study = 566/76 = 7.45
Avg. # pups per NBT X NBT cross = 6.00 (Excluding Litter #1)
Avg. # pups per Full-tail X NBT cross = 7.41
Avg. # pups per Full-tail X Full-tail cross = 7.22
(Litters with one or both parents of ‘Unknown’ tail length not included in above three averages).
Litter size of NBT X NBT crosses (6.00) is:
80.54% of 7.45 (pups per litter overall)
80.97% of 7.41 (pups per Full-tail X Full-tail + Full-tail X NBT crosses)
83.10% of 7.22 (pups per Full-tail X Full-tail cross)
Mortality:
Total # pups Overall = 566
# Pups Overall that died before 8 weeks = 38
% Mortality Overall = 6.71%
% Survival Overall = 528/566 = 93.29%
Total # Full-tailed pups = 375
# Full-tailed pups that died before 8 weeks = 28
% Mortality in Full-tailed pups = 7.47%
% Survival in Full-tail pups = 347/375 = 92.53%
Total # NBT Pups = 191
# NBT pups that died before 8 weeks = 9
% Mortality in NBT pups = 4.71%
% Survival in NBT pups = 182/191 = 95.29%
NBT X NBT litter data:
(#’s 1, 25, 40, 59, 63, 71)
Total # NBT X NBT pups = 35
Total # NBT X NBT litters in this study = 6
Avg. # pups from NBT X NBT litters = 5.83
# Surviving pups to 8 weeks = 27
% Survival in pups from NBT X NBT litters = 77.14%
NBT X NBT litter data, excluding Litter #1 (born prematurely):
(#’s 25, 40, 59, 63, 71)
Total # NBT X NBT pups = 30
Total # NBT X NBT litters in this study = 5
# Surviving pups to 8 weeks = 27
Avg. # pups from NBT X NBT litters = 6.00
% Survival in pups from NBT X NBT litters = 90%
Full-tail X NBT Litter data:
(#’s 3, 4, 5, 8b, 9, 11, 19, 22, 27, 30, 37, 41, 44, 45, 46, 48, 49, 50, 51, 54, 55, 56, 60, 64, 66, 67, 69, 70, 72, 73, 74, 75, 76)
Total # Full-tail X NBT pups = 249
Total # Full-tail X NBT litters in this study = 33
Avg. # pups from Full-tail X NBT litters = 249/33 = 7.55
# Surviving to 8 weeks = 244
% Survival in pups from Full-tail X NBT litters = 244/249 = 97.99%
Full-tail X Full-tail Litter data:
(#’s 6, 7, 8a, 12, 14, 15, 18, 20, 23, 34, 38, 39, 42, 43, 47, 52, 53, 57, 58, 61, 62, 65, 68)
Total # Full-tail X Full-tail pups = 166
Total # Full-tail X Full-tail litters in this study = 23
Avg. # pups from Full-tail X Full-tail litters = 166/23 = 7.22
# Surviving to 8 weeks = 157
% Survival in pups from Full-tail X Full-tail litters = 157/166 = 94.58%
Tail length:
% Full-tailed pups in this study = 66.25%
% NBT pups in this study = 33.75%
% No tail = 3.18%
% < ¼ = 6.18%
% ¼ - ½ = 10.78%
% ½ - ¾ = 9.01%
% ¾ - Full = 4.59%
Abnormalities:
Total # kinked tails = 14 (out of 566 pups total = 2.47%)
# NBT pups with kinks = 11 (out of 191 NBT pups = 5.76%)
# Full tail pups with kinks = 3 (out of 375 Full tail pups = .80%)
Cleft palates = 1, died before 8 weeks
Hernias = 1 diaphragmatic hernia, died before 8 weeks
Imperforate anuses = 3, died before 8 weeks
Premature births = 5 pups from 1 litter
Spina bifida or other spinal abnormalities = 0
Larger data pools present a more accurate representation of the actual population. This was a voluntary sampling of the Australian Shepherd population and consisted of just 76 litters. The Australian Shepherd Club of America’s Business Office reported registering 286 litters in September 2005 and 201 in October 2005 alone. Therefore, this study portrays only a small percentage of the Australian Shepherd litters being born. Furthermore, the proportion of NBT parents represented in this study may not necessarily be indicative of the actual Australian Shepherd population worldwide. It’s possible this study has a higher than normal percentage of bobtailed pups and parents, attributable to the title and nature of the study. Study results had 33.75% of the pups being born as NBTs, whereas CA Sharp has given me a figure of closer to 20% of pups in ASCA litter registrations being recorded as NBTs (per the ASCA Business Office database). Even though the contributed litter data may be skewed more towards litters or parents with NBTs, it also gives us a better idea of what occurs in such litters.
Because many different litter owners, many of who may have had varying interpretations of tail length, submitted data, tail length categories (none, < ¼, ¼ - ½, etc.) are approximations only. Also, the tail length at birth of each parent depended entirely upon the accuracy of the records of the original breeder of that sire or dam. For many litters, the exact tail length of the parents at birth was not known.
In summary, no spinal defects, such as spina bifida, were reported in the course of this study. One cleft palate, one diaphragmatic hernia and 14 puppies with kinked tails were reported in the 76 litters. In one of those litters (Litter #22), one of the parents had been born with a kinked tail and subsequently that litter contained a total of five puppies with kinked tails. There was a greater tendency for these kinks to occur in NBT pups vs. full-tail pups (kinks present in 5.76% of all NBT pups vs. kinks present in only .80% of all full-tail pups). In a follow-up on owners of litters in non-docking countries, I asked if pups with kinked tails were known to be more likely to suffer from any kind of spinal problems or were more prone to injury. Of those that answered, no problems were reported with dogs with kinked, undocked tails.
Three cases of imperforate anuses were reported. All three pups were NBTs; however, imperforate anuses have been recorded in pups of other breeds and species born with full-tails. 2 of those 3 pups were from the same bitch (NBT), produced in two different litters by two different sires (one NBT and one full-tail). This could indicate that there is a tendency for the imperforate anuses to be more highly inherited through certain dogs than others, but given the small size of the population in this study that cannot necessarily be concluded. It is very important to keep in mind that imperforate anuses occurred in only 3 out of a total of 566 puppies – .53% of the total pups in the study population and 1.6% of the NBT pups. All three of these affected pups died before eight weeks of age. In light of the many other inherited genetic diseases present in the breed that do not manifest themselves until much later, perhaps even after the dogs or their littermates have been bred, the occasional presence of this developmental defect (imperforate anus) should not prohibit any breeder from using those lines or parents again in the future.
Survival in puppies in litters with NBT parents (one or both) was comparable to that of litters with two full-tail parents. NBT pups were just as likely to survive to eight weeks of age as full-tailed puppies. The average litter size in NBT X NBT crosses was slightly lower - 80.97% the size of Full-tail X NBT crosses and 83.10% the size of Full-tail X Full-tail crosses; however, it should be kept in mind that only six NBT X NBT litters were represented in this study. If the NBT gene in Australian Shepherds is the same as it is in Pembroke Welsh Corgis, which has been genetically proven to be a dominant trait (lethal homozygous in utero), it would follow that live births would be reduced to about 75% of other litters.
In the Norwegian Pembroke Welsh Corgi study discussed in one of my previous articles, those results also showed that no spinal defects were associated with the NBT. Also, there was no notable difference in PWC litter size between NBT X NBT, Full-tail X NBT and Full-tail X Full-tail parents. Dr. Bruce Cattanach, of Steynmere Boxers in the United Kingdom, theorized that those homozygous NBT embryos that died in utero were replaced by healthy heterozygous embryos or embryos homozygous for full-tails. Again, the difference, in respect to litter size, between the results of the Norwegian study and the Australian Shepherd study could be attributable to the small sampling of six NBT X NBT litters in this study.
According to Dr. Cattanach, the same genetic test that was used in the Norwegian PWC study could be used to find out if the gene for NBT in Aussies is indeed the same. If so, it is a dominant gene; therefore, it would be expected that two genotypically true Full-tail parents when crossed would produce only full-tail puppies. In three Full-tail X Full-tail litters reported (#’s 14, 18 and 23), NBT pups were, however, produced. There are two possible explanations. One is that the tail status of one of the parents was not correctly recorded at birth. In countries where docking is routine, it would not be unlikely to dock tails and fail to properly record whether each pup was NBT or Full-tail at birth. The second explanation is that even though a pup appears to have a Full-tail, its genotype could be that of a NBT; just as a dog can appear solid in color and yet still be a (phantom) merle. In the Norwegian Pembroke Welsh Corgi tail study, it was surmised that there were modifying factors that determined the actual length of NBTs, from no tail to full tail and everything in between. In this study, there was no clear indication that and NBT parent tended to produce its same tail length in its offspring – only that a NBT parent can produce NBT pups with varying tail lengths.
The statistics derived from this study show that while litter size was slightly lower for NBT X NBT crosses, NBT pups were just as likely to survive to 8 weeks of age as full-tail puppies. The incidence of fatal congenital abnormalities was very low among the litters reported. Tail kinks occurred occasionally, but do not appear to be associated with any other spinal defects or an increased risk of injury. No other spinal defects were reported.
In conclusion, there is not significant evidence, in the form of increased risk for early deaths or quality-of-life-compromising congenital abnormalities, to warrant restricting the mating of two NBT Australian Shepherds together. If breeders feel that a particular NBT X NBT cross best suits the goals of their breeding programs, they should feel relatively safe in proceeding with such matings.
(My enormous thanks to all the breeders who took part in this study. I sincerely hope these findings will be of value to others, particularly those in non-docking countries, and provide one more tool on which to base breeding decisions. For further information, go to http://www.***).